1. Field of the Invention
The present invention relates to a creamed-solder printing machine and particularly to the art of pressing a squeegee on a screen with a sufficiently great pressing force.
2. Related Art Statement
Japanese Patent Application laid open for public inspection under Publication No. 7-76065 discloses a creamed-solder printing machine which prints creamed solder on a printed circuit board ("PCB"). In this printing machine, a squeegee is held in contact with a screen having a plurality of holes, and at least one of the two members is moved relative to each other so as to print the creamed solder on the PCB through the holes of the screen. The pressing force with which the squeegee is pressed on the screen influences the quality of the printing. If the pressing force is too small, too much creamed solder may be printed on the PCB such that the solder rises from each of respective squeegee-side openings of the holes of the screen, and the solders rising from two or more adjacent holes may be connected to each other. On the other hand, if the pressing force is too great, the squeegee may sweep away the creamed solder which has entered each of the holes of the screen, and too little amount of solder may be printed on the PCB through the each hole of the screen. In the latter case, the durability of the squeegee or the screen may be lowered.
Hence, the creamed-solder printing machine disclosed in the above-identified document can adjust the pressing force with which the squeegee is pressed on the screen, so that an appropriate (not too much or not too little) amount of creamed solder may be printed on a PCB. However, in some cases, even the prior printing machine may not be able to produce a sufficiently great pressing force. More specifically explained, the squeegee is supported by a frame such that the squeegee is movable toward, and away from, the screen, and the squeegee is subject to a load that is the total weight of members, such as a squeegee-support member, which are moved with the squeegee when the squeegee is moved toward and away from the screen. Thus, the greatest pressing force with which the squeegee is pressed on the screen may be defined by the total weight of those members, and the thus defined greatest pressing force may be insufficient in some cases. The pressing force may be increased by employing, as the members which are moved with the squeegee relative to the screen, members each having a larger size, or members each having a greater specific gravity. However, when those members are moved relative to the screen in the printing operation, a large vibration is produced because the great mass is moved. In addition, those members may interfere with other members. Thus, it is difficult to employ, as the members which are moved with the squeegee, members each having a large size, or members each having a great specific gravity.
In the above-indicated background, it is possible to employ a pressurized-air-operated cylinder device ("air cylinder") for pressing a squeegee on a screen. However, the air cylinder is provided with one or more sealing members which is or are provided between respective sliding surfaces of a piston or a piston rod and a cylinder housing. The sealing member or members produces or produce a great frictional resistance, in particular in the case where the pressure of a work fluid is high. In addition, the frictional resistance is not stable and accordingly the pressing force with which the squeegee is pressed on the screen is varied. Thus, an accurate and uniform printing is not obtained on a PCB.